Log into your account Control Panel (mydomain.com/cpanel) and go to the File Manager, and then the "WWW" or "public_html" folder.

Identify the file that you want to change permission by clicking on the file/directory link.

After selecting the link the right menu will update itself and show you a list of things you can do to the file/directory. Select "Change Permissions".

There should be three groups. Each group should have checkboxes for the permission types.
Definitions: (User=Owner, Group=Group, World=Other/Public)

1. set plain html pages to rw- for the user, no permission for the group, and r--for world (default)

2. set scripts to rwx for the user, r-x for the group, and r-x for world

3. set data/config files to rw- for the user, no permission for the group, and rw- for world

Select "Change" to finish.